300MA ADJUSTABLE HIGH PSRR CMOS LINEAR REGULATOR

2007-4-19

4/8

FSP2133

APPLICATION INFORMATION

Capacitor Selection and Regulator Stability

Similar to any low dropout regulator, the external capacitors used with the FSP2133 must be carefully selected for

regulator stability and performance.

exceed 0.5 inch. Ceramic capacitors are suitable for the FSP2133. Capacitors with larger values and lower ESR

provide better PSRR and line-transient response.

The FSP2133 is designed specifically to work with low ESR ceramic output capacitors in order to save space and

improve performance. Using an output ceramic capacitor whose value is ＞2.2μF with ESR＞5mΩ ensure stability.

ADJ Output Voltage Programming

The output voltage of the FSP2133 adjustable regulator is programmed by using an external resistor divider as

shown in figure 1. The output voltage is calculated using:

Resistor R1 and R2 should be chosen for approximately 7uA divider current. Low value resistors can be used but

offer no advantage and waste more power. Higher value should be avoided as leakage current at ADJ pin increase

the output voltage error. Cc is unnecessary when R1 or R2 ＜20K. The recommended design procedure is to

choose R2=169kΩ to set the divider current at 7uA and then calculated R1 using:

Load Transient Considerations

The figure7 shows the FSP2133 load transient response. It shows two components the output response: a DC shift

from the output impedance due to the load current change and transient response. The DC shift is quite small due to

excellent load regulation of the FSP2133. The transient spike, resulting from a step change in the load current from

1mA to 300mA, is 20mV. The ESR of the output capacitor is critical to the transient spike. A larger capacitance along

with smaller ESR results in a smaller spike.

Shutdown Input Operation

The FSP2133 is shutdown by pulling the EN input low, and is turned on by tying the EN input to VIN or leaving the

EN input floating.

Internal P-Channel Pass Transistor

The FSP2133 features a 0.75Ω P-Channel MOSFET device as a pass transistor. The P-MOS pass transistor

enables the FSP2133 to consume only 65μA of ground current during low dropout, light load, or heavy load

operations. This feature increases the battery operation life time.

Dropout Voltage

A regulator’s minimum dropout voltage determines the lowest usable supply voltage. The FSP2133 has a typical

300mV dropout voltage. In battery powered systems, this will determine the useful end-of-life battery voltage.

Current Limit and Short Circuit Protection

The FSP2133 features a current limit, which monitors and controls the gate voltage of the pass transistor. The output

current can be limited to 400mA by regulating the gate voltage. The FSP2133 also has a built-in short circuit current

limit.

Thermal Considerations

Thermal protection limits power dissipation in the FSP2133. When the junction temperature exceeds 150℃, the

OTP (Over Temperature Protection) starts the thermal shutdown and turns the pass transistor off. The pass

transistor resumes operation after the junction temperature drops below 120℃.

For continuous operation, the junction temperature should be maintained below 125℃. The power dissipation is

defined as :

The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of

surrounding airflow and temperature difference between junction and ambient. The maximum power dissipation can

be calculated by the following formula:

thermal resistance from the junction to the ambient.

It is also useful to calculated the junction temperature of the FSP2133 under a set of specific conditions. In this